In the apparatus described above, a disc-shaped cuvette rotates within the apparatus housing along the end faces of four iron cores. The iron cores are each surrounded by measuring field coils and are seated on a permanent magnet. The permanent magnet generates the required magnetic field which penetrates the cuvette. The disc-shaped cuvette itself is provided with cutouts to which, for example, oxygen is supplied. The cuvette with the cutouts is guided past the magnetized iron cores by the rotational movement. As soon as the cuvette moves the oxygen past the iron cores, the magnetic flux in the cores increases because of the paramagnetism of the oxygen. This increase is detected by the coils surrounding the cores since the flux change induces a voltage in the coils which can be further processed as a measurement signal by an evaluation device connected to the coils.
The rotation of the cuvette causes an alternating magnetic flux to occur in the cores and therefore an alternating voltage across the coil. The path of the magnetic flux starts from the permanent magnet on which the iron cores are mounted and passes through the cores and the cuvette and continues through the housing walls and finally ends again at the permanent magnet. The strong magnetic fields are necessary for a sensitive measurement signal. For this reason, a polarization of the cores occurs on the one hand, and a polarization of the surrounding wall of the apparatus on the other hand, because of the one-sided arrangement and connection between the permanent magnet and the iron cores. In this way, large mutually close surfaces of opposing magnetic polarity are produced between the housing walls and the magnetic components so that a large portion of the magnetic flux is lost because of stray fields and does not flow through the rotating cuvette. This stray component can amount to up to 50%.